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Subject: Re: [boost] [lockfree] review
From: Dave Abrahams (dave_at_[hidden])
Date: 2011-07-30 17:10:33

on Sat Jul 30 2011, Gordon Woodhull <> wrote:

> On Jul 30, 2011, at 8:40 AM, Dave Abrahams <dave_at_[hidden]> wrote:
>> on Fri Jul 29 2011, Gordon Woodhull <> wrote:
>>> On Jul 29, 2011, at 7:36 PM, Dave Abrahams <dave_at_[hidden]> wrote:
>>>> Personally I never understood how sequential consistency could be much
>>>> of a help in reasoning about multithreaded programs... but that's just
>>>> me.
>>> True, it's bad enough with sequential consistency, but without it
>>> there's no hope of understanding at all IMO.
>> I think I meant the opposite, or nearly so. How does not having
>> sequential consistency make reasoning worse?
> I must be missing something. Without it you can't depend on the order
> of operations even within one block and one thread, right?

Doesn't the (C++03) standard make some guarantees about sequence points
that make ordering well-defined in the single-threaded case, without
ever introducing the notion of sequential consistency?

> E.g. we were talking about translating concurrent pseudocode to real
> code. Without sequential consistency it might require inserting memory
> barriers and it's never clear where, or if some other architecture is
> going to need different ones.
> How is it not better to at least understand that a single piece of
> code will do what it says in the same order?

Could you show me an example of some single-threaded code that can't be
understood on the basis of the C++03 standard alone, and give some
examples of the multiple possible interpretations of its semantics?

Dave Abrahams
BoostPro Computing

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